C++14中引入std::shared_mutex
说明
C++14中引入std::shared_mutex
std::shared_mutex用于管理可转移和共享所有权的互斥对象，适用场景比较特殊：一个或多个读线程同时读取共享资源，且只有一个写线程来修改这个资源，这种情况下才能从shared_mutex获取性能优势。
- shared_lock是read lock。搭配std::shared_mutex使用，被锁后仍允许其他线程执行同样被shared_lock的代码。
//shared_lock构造函数中调用了shared_mutex的lock_shared函数explicit shared_lock(mutex_type& _Mtx): _Pmtx(_STD addressof(_Mtx)), _Owns(true) { // construct with mutex and lock shared_Mtx.lock_shared();}
- lock_guard和unique_lock是write lock。被锁后不允许其他线程执行被shared_lock或unique_lock的代码。
//lock_guard以及unique_lock构造函数中调用了shared_mutex的lock函数explicit lock_guard(_Mutex& _Mtx) : _MyMutex(_Mtx) { // construct and lock_MyMutex.lock();}
//unique_lockexplicit unique_lock(_Mutex& _Mtx) : _Pmtx(_STD addressof(_Mtx)), _Owns(false) { // construct and lock_Pmtx->lock();_Owns = true;}
- std::shared_mutex 相比于mutex多提供了一个lock_shared函数，用于读锁。
测试
#include
#define READ_THREAD_COUNT 8
#define LOOP_COUNT 5000000
class TimeConsum
{
public:TimeConsum() { m_nTimeStart = std::chrono::high_resolution_clock::now(); };operator double&& () const noexcept{auto end = std::chrono::high_resolution_clock::now();return std::chrono::duration_cast>(end - m_nTimeStart).count();}private:std::chrono::high_resolution_clock::time_point m_nTimeStart;
};using ReadLock = std::shared_lock ;
using WriteLock = std::lock_guard ;
using NormalLock = std::lock_guard;class SharedMutexCounter
{
public:SharedMutexCounter() = default;unsigned int get() const {//被mutable修饰的变量即使在const函数中也可以修改ReadLock lock(mutex);return value;}void increment() {WriteLock lock(mutex);value++;}std::string name()const{return "shared";}private://在C++中，mutable也是为了突破const的限制而设置的。//被mutable修饰的变量，将永远处于可变的状态，即使在一个const函数中。mutable std::shared_mutex mutex;unsigned int value = 0;
};class MutexCounter
{
public:MutexCounter() = default;unsigned int get() const {NormalLock lock(mutex);return value;}void increment() {NormalLock lock(mutex);value++;}std::string name()const{return "normal";}private:mutable std::mutex mutex;unsigned int value = 0;
};template
void test_mutex()
{TimeConsum cons;std::atomic_int32_t temp{ 0 };COUNTER counter;const auto writer = [&counter]() {for (int i = 0; i < LOOP_COUNT; ++i)counter.increment(); };const auto reader = [&counter, &temp]() {for (int i = 0; i < LOOP_COUNT; ++i)temp = counter.get(); };std::vectorreadlist;for (int i = 0; i < READ_THREAD_COUNT; ++i){readlist.emplace_back(reader);}thread wr(writer);for (int i = 0; i < readlist.size(); ++i){readlist[i].join();}wr.join();std::cout << "Test "<< counter.name()<<", count:[" << counter.get() << "]" << ",get:[" << temp << "]" << std::endl;std::cout << "Const time:[" << cons << "]" << std::endl;
}void shared_lock_test()
{test_mutex();test_mutex();
}//output
Test normal, count:[5000000],get:[5000000]
Const time:[3821.9]
Test shared, count:[5000000],get:[5000000]
Const time:[2274.59]
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